Normally the burner in a boiler is provided below the heat exchanger so that the flow of flue gases is upwardly through the heat exchanger to the flue of the boiler.
Downdraft boilers are, of course, also known in which the burner may be at the top of the boilers and the flue gases are conducted downwardly through the heat exchanger and then pass from the heat exchanger chamber to the flue of the boiler.
An important advantage of a downdraft boiler is that it can be significantly more compact than a boiler in which the flue gases flow upwardly through the heat exchanger.
However, a major problem with downdraft boilers, especially in a compact configuration, is that the burner may be directly juxtaposed with the uppermost tubes of the heat exchanger, thereby causing overheating of the water in those tubes, inappropriate boiling so that gas formation may impede the flow of water through the tubes, and encrustation of these tubes with deposits from the water which passes through the tubes.